It is expected that data traffic via radio communication doubles per year and that it will increase by one hundred times toward year 2020. If so, then it is a goal for the cellular network to evolve and innovate to meet demand, which means it is necessary to increase capacity and make the platform more compelling.
The evolution of the 3rd Generation Partnership Project (3GPP) standard is demonstrated by new features such as enhanced physical downlink control channel (ePDCCH) replacing PDCCH, where the control channel is no longer fixed to certain time-frequency resources, but more flexibly allocated, and the common reference signal (CRS) is at least significantly reduced, if not completely removed. System operation on the New Carrier Type (NCT) is now mostly based on Channel-State Information Reference Signal (CSI-RS) and demodulation reference signal (DM-RS) instead of CRS. Another expected new feature is standalone new carrier type (SA-NCT) which is assumed to be useful to bring more flexibility to cell deployment.
Small Cell Access
A small cell access without macro network node scenario is shown in FIG. 1. A small cell (also “picocell”) is simply a network access point with a very small coverage area and, perhaps, lower power radio signal than a standard network node. Small cells may handle only a few, sometimes only one, user equipment connection to a network. Small cells may operate with the full capability of a serving network node or they may require the overall supervision of a macro network node to provide access to a network. In the small cell only 120 scenario of FIG. 1 (without macro cell, compare FIG. 2), the user equipment (UE) 130 is expected to change its access node without handover. The major advantages are no extensive cell planning needed, and higher capacity is widely available for mobile terminals.
Another scenario features stand-alone long-term evolved (LTE) local area frequency layer and local area enhancements with an LTE macro cell present but no assistance assumed. However for this very attractive deployment there are still a few open issues.
First, how does UE tell one node's initial access signal from other nodes? For example, if all nodes transmit same primary synchronization signal/secondary synchronization signal (PSS/SSS), those signals will arrive with different timing at UE, therefore UE cannot find the correct downlink (DL) sync signal. On the other hand, if only a few small nodes are allowed to transmit PSS/SSS for cell ID, then maybe little or no interference will occur, but it might introduce a coverage problem for initial access. If different PSS/SSS signals are used for small nodes, another issue arises.
Second, how is UE handover (HO) among these small cells prevented? For such dense small cell deployment, reducing unnecessary handover is critical. It is desirable to have small cells in one region configured as one large cell, so that changing small cell to small cell is just an intra-cell radio resource configuration (RRC) reconfiguration instead of handover.
In traditional LTE design, the cell ID obtained from PSS/SSS detection serves two purposes; one is to link with a certain CRS pattern, and another is to support mobility (e.g., measurement, re-select, handover). This makes the two issues above contradict each other. The objective of a solution is to provide initial access to UE in any node of a small cell group and to avoid frequent handoff (HO) among the small cells when UE's mobility (that is, movement among the small cells) is moderate or even high.
Dense Small Cell Deployment Scenarios
A dense small cell deployment as illustrated in FIG. 2 has potential for handling the expected increase in wireless traffic. FIG. 2 depicts a macro network node 200 overseeing a collection of small cells (pico cells) 120. UE 130 within the small cell coverage area is handled by the small cell 120 having the best signal relation to the UE 130. According to some theorists there can be 1:1 ratio between serving nodes (small cells) and active users. In such dense deployment, it is highly desired that cell planning efforts can be reduced. At the same time, cell identification and interference mitigation need to be considered to maintain the gain from small cell deployment.
Though small cell deployment had been considered as promising to meet the capacity requirement to the network and is seen as helpful for data offloading, the dense small cell deployment also brings many problems to be solved. As shown in FIG. 2, the small cells 120 are geographically separated from the macro evolved node B (eNB) and in most case are unsynchronized to the macro cell. To enable synchronization in this scenario, the synchronization signal, PSS/SSS, is required to be sent by both macro and pico cells. Since the inter-site distance (ISD) between the pico cells can be very small, the PSS/SSS interference from other pico cells is a problem to be considered in this scenario. Beside the small ISD, there is the possibility that pico eNB has a different transmission power level, typically 30 to 37 dbm or even 24 dbm. Different Cellular Radio Exchange (CRE) bias values may be applied for neighboring pico eNBs, but it may aggravate the interference of PSS/SSS from the neighboring pico cells.
Another problem is that for mobility and for interference mitigation, the UE needs to report to the serving eNB a measured signal power and the Physical Cell Identity (PCI) for each detected neighbor cell. If two neighbor cells are with same PCI, then they cannot be distinguished. Then the allocation of the PCI has to meet the following principles:                Collision free, which means cells with coverage overlap should not have the same PCI, otherwise, the mobile terminal (MT) located in the common coverage of the two cells may not be able to decode the channels of the serving base station;        Confusion free, which means two or more neighbors of one serving cell should not have the same PCI, otherwise the serving eNB may not be able to determine the target base station (BS) during handover.Assuming the same PSS/SSS design as in the current LTE specification (Rel. 11), there are a total of 504 PCIs available. In dense small cell deployment, the collision rate of the PCI and PSS/SSS may increase and cause an interference problem. To reduce cell planning efforts, the PSS/SSS can be chosen by cells after some advanced detection on the PSS/SSS resource utilized by neighbor cells. However, due to the coverage limitation, another cell using the same PSS/SSS may not be detected by one eNB, but it can cause interference to cell-edge UEs. PCI can be derived from the PSS/SSS sequence and be used in physical layer for initialization of many sequence generation (CRS, digital reference signals (DRS), physical uplink control channel (PUCCH) resequence, etc.) but accidental PCI collision may result in interference on other signals.        